This invention is directed to redox reaction catalysts, and more particularly to improved catalysts effective for the reduction of molecular oxygen in the conduct of oxidation reactions. The invention more particularly relates to the preparation of secondary amines by catalytic oxidative cleavage of tertiary amines, e.g., the preparation of N-(phosphonomethyl)glycine by catalytic oxidation of N-(phosphonomethyl)iminodiacetic acid.
N-(phosphonomethyl)glycine (known in the agricultural chemical industry as “glyphosate”) is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in an aqueous formulation. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz, U.S. Pat. No. 3,950,402, teaches that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as “PMIDA”) with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support:
Other by-products also may form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product; and aminomethylphosphonic acid, which is formed by the oxidation of N-(phosphonomethyl)glycine. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., “leaching”) result because under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form and both N-(phosphonomethyl)iminodiacetic acid and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal.
In U.S. Pat. No. 3,969,398, Hershman teaches that activated carbon alone, without the presence of a noble metal, may be used to effect the oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid to form N-(phosphonomethyl)glycine. In U.S. Pat. No. 4,624,937, Chou further teaches that the activity of the carbon catalyst taught by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formaldehyde by-product when used to effect the oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid. This formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as “NMG”) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Optimally, therefore, it has been suggested that the formaldehyde be simultaneously oxidized to carbon dioxide and water as the N-(phosphonomethyl)iminodiacetic acid is oxidized to N-(phosphonomethyl)glycine in a single reactor, thus giving the following reaction:
As the above teachings suggest, such a process requires the presence of both carbon (which primarily effects the oxidation of N-(phosphonomethyl)iminodiacetic acid to form N-(phosphonomethyl)glycine and formaldehyde) and a noble metal (which primarily effects the oxidation of formaldehyde to form carbon dioxide and water). Like Franz, Ramon et al. (U.S. Pat. No. 5,179,228) teach using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), however, Ramon et al. teach flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%.
Using a different approach, Felthouse (U.S. Pat. No. 4,582,650) teaches using two catalysts: (i) an activated carbon to effect the oxidation of N-(phosphonomethyl)iminodiacetic acid into N-(phosphonomethyl)glycine, and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde to carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle.
Ebner et al., in U.S. Pat. No. 6,417,133, describe a deeply reduced noble metal on carbon catalyst which is characterized by a CO desorption of less than 1.2 mmole/g, preferably less than 0.5 mmole/g, when a dry sample of the catalyst, after being heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20° to about 900° C. at a rate of about 10° C. per minute, and then at about 900° C. for about 30 minutes. The catalyst is further characterized as having a ratio of carbon atoms to oxygen atoms of at least about 20:1, preferably at least about 30:1, at the surface as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
The catalysts of U.S. Pat. No. 6,417,133 have proven to be highly advantageous and effective catalysts for the oxidation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine, and for the further oxidation of by-product formaldehyde and formic acid, and without excessive leaching of noble metal from the carbon support. It has further been discovered that these catalysts are effective in the operation of a continuous process for the production of N-(phosphonomethyl)glycine by oxidation of N-(phosphonomethyl)iminodiacetic acid.
The advent of continuous processes for the oxidation of N-(phosphonomethyl)iminodiacetic acid has created an opportunity for further improvements in productivity through the development of catalysts which accelerate the rate of oxidation of N-(phosphonomethyl)iminodiacetic acid and/or formaldehyde beyond the rates achievable with the catalysts of U.S. Pat. No. 6,417,133. Since the productivity of a continuous oxidation reactor is not constrained by the turnaround cycle of a batch reactor, any improvement in reaction kinetics translates directly into an increase in the rate of product output per unit reactor volume.
Carbon and noble metal sites on the catalysts of U.S. Pat. No. 6,417,133 are highly effective for transfer of electrons in the oxidation of N-(phosphonomethyl)iminodiacetic acid, and the noble metal sites are especially effective for this purpose in the oxidation of formaldehyde and formic acid. However, the productivity of these reactions could be enhanced if the catalyst were more effective for transfer of electrons in the concomitant reduction of molecular oxygen, which can be a rate limiting step in the overall catalytic reaction between molecular oxygen and the N-(phosphonomethyl)iminodiacetic acid, formaldehyde, and formic acid substrates.